The present invention is directed to a method for regenerating sulfur dioxide gas scrubbing solutions. More particularly, the invention is directed to an improvement in the method of reoxidizing metal salts which have been reduced by sulfur dioxide absorbed in the scrubbing solutions, whereby the solutions may be recycled and reused for further sulfur dioxide removal.
A method for removing sulfur dioxide from gas streams, such as flue gases, by scrubbing with an aqueous solution of a reducible metal salt, such as ferric sulfate, has previously been described and claimed in my U.S. Pat. No. 4,070,441 issued Jan. 24, 1978. The disclosure of that patent is incorporated herein by reference.
An essential feature of the method covered by U.S. Pat. No. 4,070,441 is the regeneration or reoxidation of the reduced metal salt formed from the chemical reaction with sulfur dioxide. The regeneration makes it possible to use an initial charge of the metal salt for a long period of time by repeated recycling, without the need of discarding any spent charges and replacing with fresh ones. The regeneration of the metal salt in Flue Gas Desulfurization (F.G.D.) processes has great economic and environmental advantages over other F.G.D. methods, especially those using lime and similar materials which require dumping of the spent scrubbing material and replacement with fresh material.
According to U.S. Pat. No. 4,070,441, the regeneration of the scrubbing solution is accomplished by alternating the flue gas scrubbing with intervals of scrubbing the solution with air or other oxygen-containing gas, whereby the oxygen reoxidizes the reduced metal salt to its original high valence state. As these scrubbing cycles with flue gas and air are repeated many times, there is an increase in the sulfuric acid content of the scrubbing solution due to the oxidation and conversion of sulfur dioxide to sulfuric acid which becomes dissolved in the scrubbing solution.
It has been recognized that the rate of reoxidation of the reduced metal salt, such as ferrous sulfate, by air is impeded or slowed considerably as the sulfuric acid concentration of the solution increases. As a result, sulfuric acid build up presents a very serious obstacle to F.G.D. processes using metal salts such as ferric sulfate, and various means of accelerating the rate of regeneration have been proposed.
In my U.S. Pat. No. 4,091,075, issued May 23, 1978, it is proposed to use aluminum ions to accelerate the rate of regeneration or reoxidation of the reduced metal salt with air. The disclosure of that patent is incorporated herein by reference. Nevertheless, the depressing effect of increasing sulfuric acid content upon the rate of regeneration of the reduced metal salt by air still remains a serious factor, and more efficient methods of regeneration are being sought.
A number of other means of accelerating the rate of regeneration have been tried or proposed, including thermal acceleration, chemical accelerators and increase of oxygen in closed systems. Chemical accelerators, aside from their added expense, also have the disadvantage that their residues remaining in the solution will affect the potential subsequent industrial utilization of the sulfuric acid building up in the solution. Direct oxidizing agents such as hydrogen peroxide, ozone, etc., are volatile and can be expelled by heating the fluid. However, they are expensive and add complications to the process, as in the case with the other above described accelerating means, as well as raising some potentially serious environmental problems.